Fluvial response to tectonic activity

Lunch today was quite fun; we had a “lunch and learn” that involved a lot of really tasty brownies and an hour-long talk by John M Holbrook. It was for the most part a general overview of the surface process quirks of meandering rivers and how those affect their usefulness as reservoirs. Most basically, oil companies like drilling in point bars since that’s where the best sand packages are to be found. Understanding the complex ways that rivers meander and stack up point bars over time (and that coarse channel fill can act as a fluid transmitter between sets of point bars at times) is a way to try to maximize drilling effectiveness.

Not that mapping out historical meanders is an easy task even for a river that’s still active and not buried hundreds of feet down and only visible via cores or seismic. Dr. Holbrook used a particular section of the Mississippi River (where it crosses the New Madrid seismic zone) to illustrate these concepts, and said that when they were trying to map out the old meanders by taking sediment cores, they were wrong about 30% of the time.

I thought the most interesting part of his talk was a brief look at fluvial response to tectonic activity, particularly how a river reacts to displacement on a fault that it crosses, since that normally means a change in grade on both sides of the fault. Looking at the Mississippi’s reaction to the displacement on the faults it crossed, the basic response was for the river to straighten out (cutting off meanders) on the downdropped side of the fault (where the gradient decreases) and resume meandering on the uplifted side. Which makes a lot of sense, really, though the other interesting thing was how quickly this response occurs. (Answer: very quickly.)

He’s got a paper in the pipe1 about using the Mississippi to examine tectonic activity on the New Madrid fault, which is currently in review for Tectonophysics. Apparently it’s a bit controversial since what the river seems to show is that earthquakes along the fault system are temporally clustered, which doesn’t necessarily fit with the current consensus on the seismic zone. So I hope that it does get published and I can find a way to get my hands on the paper, since it sounds like an interesting read.

1 – If it gets published, here’s the title: Restored river courses reveal millennial-scale temporal clustering on a midplate fault

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